1. Field of the Invention
The present invention relates to an agricultural tractor having a rear suspension and in particular a draft compensating rear suspension in which the suspension is designed to reduce or minimize the effect of draft loads on the tractor attitude by minimizing compression or extension of the rear suspension.
2. Description of the Related Art
An agricultural tractor is intended primarily for off-road usage and is designed primarily to supply power to agricultural implements. An agricultural tractor propels itself and provides a draft force in the direction of travel to enable an attached, soil engaging, implement to perform its intended function. Furthermore, an agricultural tractor may provide mechanical, hydraulic and/or electrical power to the implement. Agricultural tractors must be designed with sufficient normal force, i.e. a down force, acting on the drive wheels to produce the needed draft force. Agricultural tractors differ from cargo carrying vehicles, such as pickup trucks and semi-truck tractors, in that trucks are not designed to produce a continuous draft load. A truck only needs to produce a draft load during periods of acceleration and deceleration and relies on the weight of the cargo to produce the normal force on the drive wheels.
There is a continuous desire to increase the productivity of agricultural tractors. Productivity can be increased by tractor designs that maintain tire-soil contact force when traversing uneven terrain, both during field operations and during road transport. To maintain tire-soil contact force on an uneven terrain, it is necessary to provide a suspension system to allow the tires to follow the terrain. When a suspension is added to an agricultural tractor, between the chassis and the wheels, with the hitch on the suspended chassis, an undesirable interaction occurs between the suspension and draft load. The draft load reaction through the suspension tends to force the rear suspension into jounce, i.e. the suspension compresses, causing the wheels to move upward relative to the suspended chassis. As the suspension compresses, the pulled implement, such as a plow, runs deeper, increasing the draft load. The higher the draft load, the more the rear suspension compresses until it is fully compressed. Once fully compressed, the suspension no longer provides a benefit. If the suspension is only partially compressed, it will still reduce the amount of suspension travel available to maintain tire-soil contact force and to improve the tractor ride. Suspension compression also affects the clearance under the vehicle, the height of the drawbar and hitch above the ground plane, and the attitude of the tractor.
The interaction between the suspension and draft load can interfere with the proper operation of hitch controls. When the hitch control senses the need to reduce the depth of the implement in the ground, the suspension compresses, instead of raising the implement. The reverse is true if the hitch control senses the need to lower the implement.
One way to overcome the problem of suspension compression is to provide a system that compensates for the draft load by extending the suspension. This is accomplished by adding hydraulic fluid to the system to return the suspension to a center position, whereby suspension travel in each direction is still available. However, this significantly increases the spring rate of the suspension under a draft load, resulting in a harsher ride. Furthermore, adding and removing fluid to a circuit with an accumulator results in a large energy waste.
Another solution is to provide a rigid beam axle mounted to the chassis through the suspension and mount the hitch to the unsuspended beam axle. This avoids reacting of the draft load through the suspension. However, the implement will not receive the benefit of the suspension and will follow the motion of the axle.
Yet another alternative is to provide a means to lock out the suspension when performing draft work. This eliminates all benefits of the suspension.
All of the above solutions to the interaction of the suspension to the draft load reduce the effectiveness and benefit of the rear suspension.
The present invention relates to an agricultural tractor having a rear suspension with draft compensating geometry to control the compression of the rear suspension in response to a draft load. The draft load is applied to the chassis by a ground engaging implement. The tractive load and torque at the rear wheels are reacted through the side view instantaneous center of the rear suspension. The placement of the instantaneous center affects the chassis reaction to the wheel load and torque. Locating the suspension instantaneous center on a line of 100% draft-compensation will eliminate the motion of the suspension due to the draft load. If the instantaneous center is located below the 100% draft-compensation line, the suspension will compress under a draft load, whereas if the instantaneous center is located above the 100% draft-compensation line, the suspension will extend under a draft load. The distance between the instantaneous center and the 100% draft compensation line will determine the amount of compression or extension of the suspension. An expected response to the draft load is to compress the suspension. In a tractor without a suspension, the draft load typically compresses the rear tires. By properly locating the instantaneous center of the suspension, an opposing force is created to counteract, completely or partially, the suspension compression.
The draft compensating suspension geometry can be used to eliminate or reduce the amount of suspension travel used to react to the draft load. As a result, the suspension will have travel available to react to irregular terrain profiles, which provides the operator with better ride and control. It also permits the suspension to be less stiff to terrain inputs. A draft compensating suspension also reduces the interaction between the suspension and the hitch draft control system. A draft compensating geometry eliminates the need for complex control systems to control suspension height and vehicle attitude.